The problem of balancing grinding wheels of grinding machines is known from time. Undesired vibrations are generally present in a grinding machine, and generated by out-of-balance conditions of the grinding wheel due to various possible reasons depending on the grinding wheel itself, like shape and/or constitution defects (inhomogeneity of the materials, concentricity errors between the external abrading surface and the internal centering hole, etc.), inaccurate assembling to the rotating spindle (hence causing the wheel center of gravity to be spaced apart from the axis of rotation), and, in general, deteriorations due to wear and/or splinter occurring during the machining of the workpieces. These vibrations may cause inaccuracies in the features of the machined workpieces—more specifically roundness errors like ovality and/or lobing—and introduce loads and stresses that may damage the machine tool.
Known balancing apparatuses, or balancers, are coupled to the grinding wheel and comprise movable masses, driven by electric motors that adjust the position of the masses, in the course of the wheel rotation, along radial or angular paths in order to compensate the previously mentioned out-of-balance conditions. The driving motors are also part of the apparatus, rotate along with it and the grinding wheel, and are power supplied and controlled by a stationary external power source, by means of an electric connection, including, for example, a brush collector and slip rings, or by means of a contactless connection, for example of the inductive type.
The characteristics (like, for example, the amplitude) of the vibrations generated as a consequence of the out-of-balance are picked up by processing the signals provided by an appropriate sensor and displayed, or processed in a proper unit (that often comprises the previously mentioned power supply source, too) for providing suitable balancing signals and for controlling the motors to drive the movable masses.
A balancing apparatus comprising the previously mentioned characteristics is disclosed in U.S. patent U.S. Pat. No. 3,698,263. The automatic balancing of grinding wheels generally takes place in a heuristic way, i.e. on the basis of cycles for displacing the two masses in the balancing device in order to continuously reduce the vibration detected by the sensor until there is obtained its complete elimination or in any case its reduction to acceptable values. For example, it is possible to control, in the course of the rotation, angular displacements of the masses, taken together and/or individually, in a direction and/or in the other, on the basis of the trend of the signal of the vibration-detecting sensor, until said signal reaches zero or a value very close to zero.
Automatic balancings using deterministic criteria—i.e. balancing procedures including the performing of test cycles allowing to calculate the arrangement (for example the angular arrangement) of the masses that is able to balance the system—are more difficult and expensive to apply to rotors of machine tools, as it is required to continuously check the arrangement (for example the angular arrangement) of the movable masses in the device. Deterministic procedures are more frequent in the manually-operated balancings, for example for balancing automobile wheels, where balancing weights are added by an operator—at angular positions indicated by visual indicators—for reducing the static out-of-balance. A deterministic procedure of this type is described in patent U.S. Pat. No. 4,854,168. However, should the longitudinal layout dimensions of the grinding wheel be greater than, or even comparable with, its diametral dimensions (as in the case of grinding wheels of centerless grinding machines), or should the machine tool have two grinding wheels coupled to the same shaft, at longitudinally spaced out positions, a single balancing device that operates, in substance, along a transversal plane is not sufficient for achieving the appropriate balancing. In fact, besides the so-called “static” out-of-balance at a specific area, it is not possible to disregard the dynamic couple balancing, caused by the fact that the axis of rotation that does not coincide, in general, with the axis of inertia of the rotating system. In similar cases it is known to utilize two balancing devices, substantially identical to each other and arranged at longitudinally separate positions, for carrying out a dynamic balancing of the grinding wheel, or of the grinding wheels/shaft system.
Patent application DE-A-2345664, which the preamble of claims 1 and 8 of the present patent application refers to, describes a method and an associated apparatus for the dynamic balancing of an elongate grinding wheel for a centerless grinding machine, along two transversal sections relative to the longitudinal axis, by means of two devices each including a vibration-detecting sensor and an associated unit with movable masses, each device being arranged at one of the two sections. Processing, power supply and control units are alternately connected to the two balancing devices, for controlling in sequence displacements of the masses of each of the devices on the basis of the signal received by the associated vibration-detecting sensor.
In general terms the apparatus and the method described in the German patent application do not achieve satisfying results. In fact, it has been realized that although every balancing operation at each of the two sections determines displacements of the associated masses such that the vibrations detected by the associated sensor are reduced to zero or to a minimum, the displacement of the masses at a section modifies, in general, the vibrations detected at the other section, in a way that directly depends on the elasticity of the support structure of the rotating mass, more specifically on the transversal elasticity to rotation. For this reason the apparatus and the method according to patent application DE-A-2345664 provide acceptable results only if applied to structures with very particular features but, in general terms, do not enable to overcome the problem of the correct dynamic balancing of rotating systems.